1. Field of the Invention
The present invention relates to a disk device, and a disk medium, in which a plurality of servo cylinders formed concentrically from the inner diametrical portion to the outer diametrical portion of at least one disk, such as a magnetic disk, are divided into several areas, and a different servo signal frequency is set for each of these areas and recorded in advance.
A disk rotatably set on a disk device, such as a magnetic disk device, may be removable from the disk device as in the case in which it is used as a single sheet of a disk of servo track writer (STW). In such a case, the disk may be shipped independently with servo signal patterns written in the servo cylinders. This disk (or a plurality of disks) is generally called “a disk medium” (or “disk media”).
2. Description of the Related Art
In order to facilitate the understanding of the problems of a disk device with servo signal patterns arranged according to the prior art, the configuration and the operation of a disk device with ordinary servo signal patterns arranged thereon will be explained with reference to FIGS. 1 to 6 that will be described later in the section “BRIEF DESCRIPTION OF THE DRAWINGS” herein.
A schematic diagram, showing an example of an arrangement of a conventional servo signal patterns, is illustrated in FIG. 1. Further, a diagram showing the relationship between magnetization reversal patterns of the servo signal recorded with a predetermined servo signal frequency and the reproduced servo signal waveform, is illustrated in FIG. 2.
As shown in FIG. 1, the servo signal pattern SSP of a conventional magnetic disk device is generally recorded with a predetermined fundamental servo signal frequency in a plurality of servo cylinders 200 formed concentrically from the inner diametrical portion to the outer diametrical portion of a disk 100. As a result, each area of the servo signal patterns is arranged sectorially on the disk 10.
The “the servo cylinder” is defined as an aggregate of a plurality of servo tracks (i.e., a cylinder of a plurality of servo tracks) of a plurality of disks arranged in a stacked form, on which the servo signal patterns are embedded in advance and which are arranged in vertical direction so that an access to these servo tracks can be made simultaneously by using a plurality of read/write heads.
The disks 100 of the magnetic disk device are rotated at a constant rate and, therefore, the distance in which each disk is moved by the rotation of each disk in the outer diametrical portion of each disk (outer side) is longer than that in the inner diametrical portion (inner side) of each disk, within the same time length. In the case in which servo signal patterns of the same number of bits are recorded with a predetermined servo signal frequency by using a read/write head 500 suspended by an arm 400, therefore, the servo signal pattern on the outer side occupies a larger area than that on the inner side. In other words, the recording density of the servo signal pattern on the outer side tends to be smaller than the recording density of the servo signal pattern on the inner side.
In view of this, a variable servo signal frequency recording system described later has been conceived of as a conventional technique in which the recording operation is carried out with a higher servo signal frequency for the servo cylinders on the outer side than that for the servo cylinders on the inner side thereby to relatively reduce the area occupied by the servo signal pattern on the outer side.
Generally, in the case in which the servo signal patterns are recorded with a predetermined servo signal frequency for a plurality of servo cylinders on the surface of the disk, as shown in FIG. 2, the reproduced servo signals each having the different waveform are generated from the magnetization reversal patterns of the servo signal, even though the same servo signal frequency is used, among the outer side (outer diametrical portion), the center side (intermediate portion) and the inner side (inner diametrical portion). The reason for this is that, in fact, the recording density of the servo signal patterns is relatively high on the inner side and relatively low on the outer side, so that the waveforms of the reproduced servo signals are different from each other, in spite of the fact that the same servo signal patterns are written.
In the read channel for demodulating the servo information from the servo signal waveforms, therefore, as shown in the graph of FIG. 3, the servo signal error rate of the gray code having the servo cylinder information in an ordinate of the graph is varied with the recording density of the servo signal patterns (i.e., servo signal pattern density in an abscissa of the graph). In order to use a disk device having a comparatively superior characteristic of the servo signal error rate of the gray code, the recording density of the servo signal patterns is required to be substantially constant.
From this viewpoint, the employment of a conventional system of a variable servo signal frequency type, in which the servo signal patterns are recorded by changing the servo signal frequency from the inner side of the disk to the outer side of the disk, is expected to maintain a substantially constant recording density of the servo signal patterns and thus to improve the servo signal error rate of the gray code.
A schematic diagram showing an example of a layout of the servo signal patterns used in the prior art described above, is illustrated in FIG. 4. Among various examples according to the prior art, examples of the layout of the servo signal patterns, in which some representative conventional systems of a variable servo signal frequency type are employed, will be described below (please refer to Japanese Unexamined Patent Publication (Kokai) Nos. 3-130968, 5-174516, 5-94674, and 10-255416 and Japanese Patent No. 2973247, if necessary). In the example shown in FIG. 4, a plurality of servo cylinders 200 of a disk 100 are divided into three areas (PP0, PP1 and PP2) from the inner side to the outer side, and servo signal patterns are recorded with different servo signal frequencies (Fs0, Fs1 and Fs2) in each area.
A schematic diagram showing an example of the locus of the read/write head, in which the estimated position of the read/write head contains an error, according to the conventional system of a variable servo signal frequency type, is illustrated in FIG. 5. Further, a diagram showing the reproduced servo signal waveforms in the boundaries between different servo signal patterns according to the conventional system of a variable servo signal frequency type, is illustrated in FIG. 6.
As far as the function of the currently-employed servo signal demodulation for the read channel is concerned, any other servo signals than the servo signal of the given servo signal frequency which is set in advance so as to have the function of the servo signal demodulation, cannot be accurately demodulated. Although there also exists a read channel that is capable of demodulating a servo signal of a servo signal frequency deviating from the given servo signal frequency which is set in advance so as to have the function of servo signal demodulation, the tolerable range of the frequency deviation from the given servo signal frequency is very narrow.
In the case in which the conventional system of a variable servo signal frequency type is employed, therefore, the servo signal frequency for the read channel is required to be set beforehand, with respect to the change in the servo signal frequency of the given servo signal pattern read by the read/write head 500. In order to set the servo signal frequency beforehand, the position of the read head at the time of reading the next servo signal pattern is estimated by using the servo information thus far demodulated.
When the read/write head carries out the seek operation, the position of the read/write head at the time of reading the next servo signal pattern is estimated. No problem would occur if the estimated position is always estimated accurately.
Assume that the value of the estimated position contains an error such that, for example, the estimated position of the read/write head is in the neighborhood of the boundary (B01) between the area PP0 of the servo signal pattern written with the first servo signal frequency Fs0 and the area PP1 of the servo signal pattern written with the second servo signal frequency Fs1. In the case in which the area actually reached by the read/write head and the area associated with the estimated position of the read/write head are different from each other, a servo signal frequency different from the servo signal frequency of the area actually reached by the read/write head is undesirably set for the read channel, thereby leading to the problem that it becomes impossible to demodulate the servo information.
More specifically, while several servo signal patterns are read during a low-speed seek operation, assume that the read/write head passes through the neighborhood of the boundary between the area of the servo signal pattern written with the first servo signal frequency and the area of the servo signal pattern written with the second servo signal frequency adjoining the first area. In the case in which the estimated position of the read/write head contains an error, the area of the servo signal frequency associated with the estimated position of the read/write head and the area of the servo signal frequency associated with the actual position of the read/write head are different from each other. Thus, the setting of the servo signal frequency of the read channel is different from the servo signal frequency of the servo signal actually read, and therefore, the servo signal cannot be demodulated.
Further, when reading the next servo signal pattern, the position of the read/write head is estimated in the absence of accurate position information about the read/write head for the latest read operation. Therefore, the estimated locus containing an error is estimated again. As a result, an inaccurate servo signal frequency, which is different from that associated with the actual position of the read/write head, is set.
Assume that the actual position of the read/write head and the estimated position of the read/write head is moved as shown in FIG. 5. Once the servo signal frequency for the read channel is erroneously set to the value for the area different from the actual position of the read/write head, the servo signal frequency continuously fails to be set and the servo signal often cannot be demodulated, thereby causing a seek error undesirably.
When the read/write head is positioned on the track (i.e., positioned in an on-track condition) in the boundary between the areas of the servo signal patterns written with different servo signal frequencies, the servo signal patterns of the adjoining different servo signal frequencies are read simultaneously. Therefore, the waveform read by the read/write head, as shown in FIG. 6, is composed by a combination of the reproduced servo waveforms of the adjoining two areas.
In the case in which the servo signal frequency Fs0 is set for the read channel at this time, the component of the servo signal written with the servo signal frequency Fs1 in the servo signals that have been read makes up noise. This noise is greatly correlated with the servo signal of the servo signal frequency Fs0. Thus, the error rate of the servo signal of the servo signal frequency Fs0 is deteriorated, and demodulation errors frequently occurs, thereby leading to the problem that the read/write head cannot be positioned on the track in an accurate position.